It is essential that certain equipment, such as, signal, control and communication apparatus for railroad and mass and/or rapid transit installations, operate in a fail-safe manner. In order to ensure safe operation, it is necessary to meticulously analyze and carefully examine each device, component, circuit and network for all possible and conceivable failures. In addition to changes in the component parameters and circuit characteristics which occur due to temperature variations and due to deterioration with age, there are two common dramatic malfunctions, namely, a short circuit and open circuit, which must be positively protected against in order to prevent an unsafe condition. For example, it is mandatory to fully consider how minute changes in the values of both passive and active elements will effect the operation of the circuits and overall system, and also what adverse results will occur when a resistor, capacitor, inductor, diode, transistor, etc. becomes short or open circuited. It is readily apparent that a further prerequisite in the vital operation of electrical and electronic circuits and apparatus is a suitable voltage supply source. In the past, it was common practice to employ an appropriate voltage responsive device or means for regulating and stabilizing the voltage level of the power supply. In keeping with fail-safe design principles, it must be assumed that the amplitude of the regulated voltage will remain substantially constant and that under no circumstance will the voltage increase in value. Thus, a circuit or component failure in a fail-safe voltage regulator must not be capable of causing a rise in the supply voltage. In previous types of shunt regulators employing conventional voltage breakdown devices an unsafe condition prevailed whenever the breakdown device failed in an open circuit manner. That is, in prior art regulators a poor or high resistance solder joint, a loss of a connection or lead or an open circuit failure of the voltage breakdown device results in the loss of regulation and causes a marked increase in the voltage level. The loss of regulation is intolerable in a fail-safe operation since increased voltage levels are capable of causing erratic operation and critical circuit malfunctions which can impair the security and integrity of the overall system. Thus, previously known voltage regulators are not suitable and are not generally recommended for use in vital types of transit operations since the appearance of a critical failure can jeopardize the safety of individuals and can result in severe damage to the equipment.